A Short History of Measurement Units

Measuring with the body

The earliest units came from what people had to hand. The cubit was based on the length of the forearm, the foot on a foot, and the pace on a stride. The English word mile descends from the Latin mille passus, a thousand paces, and the acre was traditionally the area a team of oxen could plough in a day. These units were practical because everyone carried a reference with them.

They were also inconsistent. Forearms and feet vary, so a cubit in one city differed from a cubit in another. Ancient civilisations such as Egypt maintained official standards, often kept as physical rods, but the shared understanding of a unit usually stopped at the edge of a kingdom or a trade guild.

Specialised fields kept their own habits too. At sea, speed was once measured by paying out a knotted line behind a ship and counting the knots that passed in a set time. That is where the knot comes from, and the unit survives today as one nautical mile per hour.

A patchwork of local standards

For centuries a merchant crossing a border had to learn a new set of measures. Before the French Revolution, France alone had a great many local units whose names were shared while their sizes differed from town to town. This made trade, taxation and science harder, because a number only meant something if you knew whose pound or whose foot was meant.

Britain developed its own long-lived collection of traditional units and, in 1824, standardised them in the Weights and Measures Act, creating what we now call imperial units. The result was a consistent system within the British Empire, but one with awkward ratios such as 12 inches to a foot, 3 feet to a yard and 1,760 yards to a mile. Temperature scales also date from this period of invention: Fahrenheit proposed his scale in the 1720s and Celsius his in the 1740s.

The metric idea

In 1791 a report to the French National Assembly outlined a decimal system built on the metre and the kilogram. The metre was to equal one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris, and surveyors, led by Jean-Baptiste Delambre and Pierre Méchain, spent years measuring the stretch between Dunkirk and Barcelona to calculate it. The kilogram was linked to the mass of a cubic decimetre of water, which is close to a litre.

In 1799 platinum standards for the metre and kilogram were deposited in the French archives. The decisive innovation was not only the standards but the decimal structure: larger and smaller units were made by multiplying or dividing by powers of ten and attaching prefixes such as kilo and milli. That made conversion within the system a matter of moving a decimal point.

International agreement

A system only helps if many countries share it. On 20 May 1875, seventeen countries, including the United States, signed the Metre Convention, which created the International Bureau of Weights and Measures near Paris. In 1889 the new international prototypes of the metre and the kilogram were adopted; the kilogram prototype, a metal cylinder, remained the world’s definition of mass until 2019.

The United States had already made the metric system legal for use in 1866 without making it mandatory. In 1959, the United States and several Commonwealth countries agreed to define the yard as exactly 0.9144 metre and the pound as exactly 0.45359237 kilogram. That agreement is why inch-to-millimetre and pound-to-kilogram conversions are exact today.

From artefacts to constants

In 1960 the General Conference on Weights and Measures gave the modern metric system its name, the International System of Units, or SI. Definitions then moved steadily away from objects. The second, once a fraction of the day, was tied to an atomic transition in caesium in 1967. In 1983 the metre was redefined through the speed of light, fixed at 299,792,458 metres per second.

The last big step came on 20 May 2019, when the kilogram was redefined by fixing the value of the Planck constant, ending the reign of the physical prototype. Units are now tied to constants of nature, so any laboratory with the right equipment can realise them without comparing to a single object.

What history means for conversions

This history explains why modern conversion factors look the way they do. Because the inch, foot, yard and pound are now defined in terms of metric units, a conversion such as 1 inch = 25.4 mm involves no measurement error. Some traditional units, such as the nautical mile at exactly 1,852 metres, were fixed in the same way by international agreement.

A unit converter such as this one applies those fixed factors in the browser, so you can see the relationship between old and new units directly. For historical documents, remember that a pound, a gallon or a mile from earlier centuries or another place may not match today’s definitions, so check specialist sources before converting them.